The present invention relates to a method for controlling the temperature dependence of a junction barrier Schottky diode of a semiconductor material having an energy gap between the valence band and the conduction band exceeding 2 eV. No such method is known today, and the formulation of the goal to obtain such a method constitutes a part of the present invention.
A junction barrier Schottky diode is already known through U.S. Pat. No. 6,104,043. That diode is made of SiC, which belongs to the wide band gap materials to which the present invention is directed. The present invention is directed to junction barrier Schottky diodes of all materials having a said energy gap exceeding 2 eV, such as SiC, diamond, AIN etc, but it is particularly directed to such diodes of SiC, so that the present invention will hereinafter be discussed for that material so as to illuminate, but not in any way restrict, the invention and the problems to be solved thereby.
SiC has some excellent physical properties, among which a high breakdown field being approximately ten times higher than for Si may be mentioned. These properties make SiC well suited as a material for power devices operating under conditions where high voltages may occur in the blocking state of the device. Due to the large band gap of SiC a Schottky diode made of this material has particular advantages with respect to on-state losses compared to a pin-diode made of SiC, since the forward voltage drop of a pn-junction is very large for SiC as compared to for example Si. This is valid for blocking voltages below a predetermined level, above which pin-diodes of SiC have a lower on-state voltage. Furthermore, by arranging laterally spaced emitter layer regions for forming a junction barrier Schottky diode a pn-junction-behaviour will result in the blocking state of the device thanks to the pinching off of the Schottky region resulting in low leakage currents. Furthermore, such a diode and systems made with these diodes of SiC will have very low switching losses at higher frequencies thanks to the low reverse recovery charge of SiC in comparison to Si and that a Schottky diode has no minority charge carriers which have to recombine as in a pin-diode.
In some applications it is important and even necessary that the diode has a positive temperature coefficient, i.e. the on-state voltage thereacross increases with the temperature, since the current through the diode may otherwise run away and destroy the diode and possibly other equipment associated therewith. This is in particular the case when connecting a number of such diodes in parallel. This is no problem for such a diode of Si, since the resistance of the drift layer totally dominates, so that the temperature coefficient will be positive already for low current densities. However, diodes of wide band gap materials, such as SiC, require a much thinner drift layer due to the high dielectric strength of SiC, so that the contribution of the resistive voltage drop of the drift layer to the total on-state voltage will be relatively smaller than for a device of Si. Instead the Schottky barrier dominates and the contribution thereof results in a negative temperature coefficient. This means that Schottky diodes of SiC made for blocking comparatively low voltages in the blocking state thereof and accordingly being very thin, may not have a positive temperature coefficient until very high current densities, and the cross-over point where the temperature coefficient shifts from negative to positive may be as high as several hundreds of A/cm2 in the case of SiC as compared to typically 30-250 A/cm2 in the Si case. However, cooling aspects as well as encapsulation problems normally require an average current density not being much higher than 100-200 A/cm2. A solution to this problem would be to use thicker SiC diodes being dimensioned for blocking very high voltages also in cases where no such high voltages have to be blocked. However, this would mean that a great deal of the advantage of SiC would be lost and the static operation losses would increase enormously. Furthermore, such diodes would be much more costly than diodes made for lower blocking voltages.
The object of the present invention is to find a solution to the problems mentioned above.
The present inventors have understood that a junction barrier Schottky diode results in totally new possibilities to control the temperature dependence of a Schottky diode without influencing the switching losses or even the static losses thereof considerably.
Accordingly, the present invention provides a method of the type defined in the introduction, in which the following steps are carried out when producing the diode:
1) epitaxially growing on top of each other the following semi-conductor layers of said material: a substrate layer being highly doped according to a first conductivity type, n or p, and a drift layer being low doped according to said first conductivity type,
2) introducing dopants of a second conductivity type, n or p, opposite to the first conductivity type into the drift layer in regions laterally spaced to form doped emitter layer regions in the drift layer at a vertical distance from the substrate layer for defining a grid portion of the diode there, and
3) applying a metal layer on top of the drift layer to make a Schottky contact thereto and on top of at least one of the emitter layer regions to make a contact thereto,
wherein at least step 2) is carried out for adjusting the on-state resistance of the grid portion of the diode for obtaining a temperature dependence of the operation of the diode adapted to the intended use thereof.
Accordingly, the invention resides in the understanding that it is possible to control the temperature dependence of a junction barrier Schottky diode by adjusting the on-state resistance of the grid portion of the diode when producing the diode. It has been found that the resistance of the grid portion of a junction barrier Schottky diode of a wide band gap material may be adjusted in such a degree when producing the diode that the total resistance of the diode may be considerably changed, so that said cross-over point between a negative and a positive temperature coefficient may be moved drastically. Furthermore, it will be possible to change the value of the temperature coefficient in this way.
It is possible in this way to change the total resistance of a low blocking voltage junction barrier Schottky diode considerably by changing the on-state resistance of the grid portion, since it constitutes a considerable part, sometimes the major part, of the total on-state resistance. In the case of a junction barrier Schottky diode of SiC for 600-1700 V the resistive contribution from the grid portion is typically 80% of the total on-state resistance. Accordingly, it will be possible to change the on-state resistance of the diode considerably by changing the resistance of the grid portion while substantially not influencing the switching losses of the diode, since the thickness and/or the doping of the drift layer does not have to be changed. Accordingly, said cross-over point of the temperature coefficient of the diode may be lowered considerably without making the drift layer thicker.
Thus, the temperature behaviour may be optimized independently of the drift layer. This method may also be used to control the capacitive behaviour of the diode for minimizing unwanted oscillations in a circuit in which the diode may be built in. xe2x80x9cTemperature dependence of the operationxe2x80x9d as used here is accordingly intended to also cover that case.
According to a preferred embodiment of the invention the method is carried out for producing a diode to be connected in a package in parallel with other such diodes for sharing an on-state current through said package, and step 2) is carried out for adjusting the resistance of the grid portion of the diode for obtaining that the temperature coefficient of the diode will be positive or nearly positive at the intended current density and voltage blocking capability of the diode. Paralleling of many chips of such diodes is important in many applications, such as in power converters, since the size of each chip is still very small, for achieving high currents in the range of for example 100 A. As mentioned above, a positive temperature coefficient for the current density in question is often a condition for such paralleling, and this may be achieved also for low current densities for diodes with low voltage blocking capability in this way. xe2x80x9cNearly positivexe2x80x9d means that it may be possible to in this way obtain a temperature coefficient being less negative and by that use for example 250 V diodes of SiC for paralleling under certain conditions.
According to another preferred embodiment of the invention the resistance of the grid portion of the diode is in step 1) adjusted by adjusting the doping concentration of drift layer regions later in step 2) becoming a part of said grid portion. The resistance of the grid portion may in this way be increased by decreasing the doping concentration of said drift layer regions and conversely. Another positive effect of this lower doping concentration in the grid portion is a higher yield of the diodes, since other negative effects in the surface region of the diode are reduced by that.
According to another preferred embodiment of the invention the relation between the lateral cross section area of said drift layer regions of the grid portion with respect to the total lateral cross section area of the diode is in step 2) adjusted for adjusting the resistance of the grid portion. It has been found that this relation is another parameter crucial for the resistance of the grid portion and which may be changed without substantially influencing the switching losses of the diode.
According to another preferred embodiment of the invention the doping concentration of the drift layer is increased with respect to the maximum doping concentration allowed for a diode without any grid portion for lowering the resistance of the drift layer and the on-state losses of the diode at a given voltage blocking capability of the diode. This embodiment is particularly well suited for blocking voltages as of which the unipolar drift resistance is the dominating part of the on-state resistance of the diode, which for SiC means a blocking voltage of 900 V and higher. It has been found that the different blocking mechanisms in the junction barrier Schottky diode compared to a normal Schottky diode allow a higher desired critical electric field in the junction barrier Schottky diode. This means that the resistance of the drift layer may be lowered, since for a given blocking voltage the doping concentration of the drift layer may be increased and thereby the drift layer may be made thinner. This results in a lower total forward voltage of the junction barrier Schottky diode compared to a normal Schottky diode and will influence the temperature dependence of the diode.
According to another preferred embodiment of the invention said material is SiC. This means that it will be possible to benefit from all the advantageous properties of SiC in a junction barrier Schottky diode.
According to another preferred embodiment of the invention a diode having a voltage blocking capability of 600-3500 V, preferably between 600 and 1500 V, is produced.
According to still another preferred embodiment of the invention a diode having the drift layer doped by donors is produced. This seems to be the most preferred doping type for the drift layer, although the invention is not restricted thereto, but it also comprises the use of acceptors as dopants for the drift layer and thereby hole conduction.
The invention also relates to a junction barrier Schottky diode produced by carrying out a method according to the present invention.
Furthermore, the invention relates to a use of a junction barrier Schottky diode produced by carrying out a method according to the invention in a package in parallel with other such diodes for sharing a current through this package, which may for blocking voltages of 1200-1800 V be possible also for current densities being as low as 150 A/cm2 in the on-state of the diode. It will also be interesting to apply the method according to the invention on larger SiC chips with a cross section exceeding 10 mm2.
Other advantages as well as advantageous features of the invention will appear from the following description and the other dependent claims.